Magnetic resonance tomography (MR or MRT) is an imaging method for displaying tissue in the human or animal body. MRT is based on the principle of nuclear magnetic resonance in accordance with which atomic nuclei such as the hydrogen nuclei present in large numbers in the body exhibit a magnetic moment. Consequently, they can be excited in an applied external magnetic field with the aid of electromagnetic radiation in the radio frequency range (termed RF radiation or RF pulse), and output this radiation shortly thereafter. This RF radiation, also termed an MR signal, is detected with the aid of an RF antenna that can also be used to generate the exciting radiation. The external magnetic field is mostly generated by a superconducting main magnet that encloses a horizontal examination tunnel into which the object to be examined, for example a patient on a couch board, is pushed. Spatial coding of the MR signals can be achieved by magnetic fields having a spatial gradient that are generated by so-called gradient coils.
The unit surrounding the examination tunnel and composed of main magnet, gradient coils and, if appropriate, an RF antenna is designated in general as a “field generating unit”. Further components of an MR system such as control computer, ADCs, frequency generators etc are not part of the field generating unit, but are generally arranged next to the field generating unit in a separate room.
A further medical imaging method is positron emission tomography (PET). PET is particularly suitable as a nuclear medicine method for displaying biochemical processes in the body, for example for finding tumors and metastases. The patient is in this case administered a tracer with a radionuclide that is distributed in the body and outputs radioactive radiation in the process in the form of positrons. After a short time, the positrons decay into two gamma quanta that move in opposite spatial directions and are captured by suitable detectors. The latter are mostly arranged around the body as an annular PET detector. For example, the gamma radiation is captured by a matrix composed of scintillation crystals in which a strike by any photon produces a scintillation. The latter is, in turn, captured and amplified by photodetectors, for example by avalanche photodiodes.
There has recently been interest in combining MRT and PET with each other in one device, to allow both imaging modalities to be used simultaneously or one after the other in the same body part of the patient (as far as possible with the same image field). For this, a PET detector ring is also integrated into the field generating unit.
Instead of the RF antenna integrated in the field generating unit, which is also termed a whole body antenna, use is often also made of so-called local coils in order to improve the signal-to-noise ratio in the MRT. The local coils are RF antennas that are adapted to specific body parts and, after the patient has been supported on the couch board, but still outside the field generating unit, are laid by the operating staff directly on the body part to be examined. Often, the RF pulses are then still generated by the whole body antenna, while the body coil functions as receiving coil and receives the MR signals. It is thereby possible to achieve a substantially better image quality in comparison with reception with the aid of the whole body antenna installed fixedly in the field generating unit.
However, the use of local coils exhibits various disadvantages in the case of a combined MR/PET system: firstly, in order to apply the local coils, the operating staff require a time of up to five minutes per patient for positioning and later removing the local coils. By contrast with the patient examinations on PET devices currently in use, the operating staff are thereby exposed to a radiation burden lasting for a longer time period.
Moreover, the structure of the interior local coil reduces the sensitivity of the PET detector ring and requires a correction. However, a correction is scarcely possible in the case of PET image reconstruction owing to the variable spatial arrangement of the local coils as a function of the body shape of the patient to be examined.
DE 103 18 190 A1 describes an antenna arrangement for a pure MR device that is intended to replace the conventional local coils laid directly on the body. This arrangement is of lesser diameter than the whole body coil, but permanently integrated in the examination tunnel. For example, it is intended to be held with an accurate fit in the examination tunnel. The installation and dismantling of the antenna arrangement is therefore very laborious in the case of DE 103 18 190 A1, since an operator must lean from one side into the field generating unit in order to insert or remove the antenna arrangement. It is therefore not feasible to adapt to changing patient diameters.